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Ron Johnson is turning out to be a very good, very capable candidate, and the fact that he had a ton of money doesn’t hurt either,” Mr. McAdams said. “He’s done a very good job responding to and pre-empting Feingold’s attacks.”

His lack of political experience resulted in a few early campaign stumbles. During a meeting months ago with “tea party” supporters, he expressed support for the USA Patriot Act, a law enacted during the George W. Bush administration that is loathed by many in the conservative movement as an infringement of personal rights. A lack of detailed knowledge of some constitutional issues also didn’t help.

But since winning the Republican nomination in the spring, Mr. Johnson aided by a smart and savvy campaign staff has refined his message and appearance.

Mr. Johnson, soon after winning his party’s endorsement, trailed the incumbent in several polls by less than 5 percentage points even though the surveys showed a majority of voters didn’t recognize his name.

“That is stunning for a candidate to be that little known and yet run just a few points behind the incumbent,” Mr. Franklin said. “I think that’s the strongest evidence there is that voters this year were not evaluating Ron Johnson versus Russ Feingold based on what they know about Ron Johnson. They were evaluating a Republican conservative, maybe with tea party ties, versus the Democrat.”

The Democrat has embraced Mr. Obama’s agenda at times and appeared with the president at a huge Sept. 28 campaign rally in Madison, despite reports that he was planning to skip the event. One of Mr. Feingold’s campaign ad defends his vote for the health care bill and accuses Mr. Johnson of siding with insurance companies at the expense of ordinary Wisconsins.

To be competitive, Mr. Feingold must define his opponent and attack him on specific issues, analysts say.

“He’s got to find something negative to say against Ron Johnson that really sticks,” Mr. McAdams said. “I’m not that sure what that would be, so I think the odds are pretty strongly that Ron Johnson becomes senator.”

But the Feingold camp also has suffered a few embarrassments. The campaign was forced to pull and re-edit a TV ad last week when it included a small but unauthorized video clip from a National Football League game. Last month, the senator was admonished for using a clip from a Madison, Wis., TV station without permission.

Dean Debnam of Public Policy Polling said there is still time for Mr. Feingold to recover, although he said the incumbent must find and exploit a significant weakness in the Johnson campaign. One problem: His Republican rival does not have the long trail of colorful and provocative statements on issues that tea-party-backed newcomers have had to defend in other races across the country.

“What I’ve been saying to Democratic incumbents is, the bad news is it’s an anti-incumbent year; the good news is it’s not a pro-crazy year, except that [Mr. Johnson] hasn’t put himself in the far crazy position like [Delaware Republican Senate nominee] Christine O’Donnell,” said Mr. Debnam. “So it doesn’t give [Mr. Feingold] as much room to move.”